The rotary internal combustion engine has a unique dynamic thermal problem because the epitrochoid surface of the rotor housing experiences wide variations in temperatures at different locations therealong. This is due in part to the fact that the three combustion chambers are fired at the same location by the same spark plugs resulting in an extremely high surface temperature in the localized area about the firing point. In fact, this localized area runs considerably higher in temperature than a reciprocating engine cylinder barrel. To further amplify the temperature problem, the same intake port operates to charge each of the three combustion chambers resulting in an exceedingly low epitrochoid surface temperature near such intake port. It has been found through test analysis of a commercial engine, such as the Mazda type now commercially available, that the temperature variation of the epitrochoid surface was exceedingly wide; it varied from 208.degree.F to 460.degree.F at 6,000 r.p.m. (wide-open throttle). The cooling system of the commercial Mazda type engine is of the type where the cooling water traverses each of the housings to complete a circuit.
This wide temperature variation leads to improper lubrication between the apex seal and the epitrochoid surface. In those areas where the low temperatures are experienced along the epitrochoid surface, the oil lubricant tends to ball-up ahead of the apex seal. In those areas where high temperatures are experienced, the lubricant cannot keep the epitrochoid surface sufficiently wet because of the excessive surface temperature; this results in a high friction condition between the apex seal and the epitrochoid surface which induces "chatter". Chatter is a phenomenon resulting from the apex seal momentarily leaving the epitrochoid surface and then returning to dig into the surface. A rotor housing having a high degree of chatter will show closely spaced transverse lines or grooves in certain quadrants of the rotor housing. Obviously, the grooves themselves are a detriment to proper sealing.
Yet still another thermodynamic problem, unique to the rotary internal combustion, is the high amount of unused heat that is rejected by the engine. Because of the general geometry of a rotary internal combustion engine, as compared to an average reciprocating engine, the large surface-to-volume ratio of the combustion chamber results in a theoretical transmission of considerably greater amounts of heat to the coolant and/or oil. Heat rejection tests confirm this fact. The rotary internal combustion engine also has a longer expansion stroke than a similar reciprocating engine. In some respects, this is a good factor because it eliminates torque reversals in the engine and the engine runs much smoother. On the negative side, the longer expansion stroke permits a greater amount of time for heat to be transmitted through the walls of the combustion chamber.
Materials Selection has been hampered by the thermodynamic problems. The uneven temperature distribution about the epitrochoid surface is one of the road blocks to utilization of cast iron for the rotor housing. Typically, aluminum, because of its excellent heat transfer qualities, has been the only material used for rotor housings. But under severe operating conditions and events, certain structural changes can occur in the aluminum as a result of the uneven temperature distribution: (a) repeated thermocycling can result in cracks around the spark plug holes, (b) distorted housings can deteriorate the gas, oil and coolant sealing systems, and (c) in an engine equipped with an aluminum rotor housing sandwiched between cast iron side housings collapse of the aluminum housing could occur. The temperature danger level appears to be approximately 400.degree.F for the above problems to occur.
Any solution to the thermodynamic problem must contend with the type of cooling systems used by the prior art. The most prevalent prior art engine used commercially is that of the current Mazda engine which employs an axial-flow cooling system. The term axial flow is used here to mean that the fluid enters at one side wall of the composite of housings and moves through each housing. Fluid transfers to the next housing in an axial direction parallel to the axis of the rotary shaft of the engine. This necessitates that the five major housings (to constitute a two rotor engine) be sealed on the four main surfaces to prevent coolant leaks externally and internally. The housings of the Mazda engine employ a large number of O-rings, eight of which are very large. These five housings also require the use of an extremely large number of tie bolts to hold the housings securely together and to compress the O-rings for preventing leakage. The bolts are extremely long and after torquing these bolts to a final installed condition, excessive twisting may occur making it very difficult to maintain proper compressive force.
Several engines of the Mazda type have failed in hot and cold cycling tests and particularly in the O-ring seal area because of inadequate torque on the tie bolts. All of the problems associated with uneven heat transfer outlined in previous paragraphs are experienced with the Mazda engine.
Other prior art cooling systems have included peripheral flow of the cooling fluid within a single housing, such as an end housing or a rotor housing. But in almost all cases the definition of the housing cooling circuit has required the use of end plates to complete the internal flow passages and these plates in turn required sealing members reverting back to the original problems of the Mazda engine.
Some attempt has been made by the prior art to vary the cooling capacity of the cooling circuit in an axial flow or peripheral flow arrangement. These attempts have been primarily on a mathematical concept basis and have not developed suitable practical implementation to obtain the desired goals as set forth by such mathematical analysis. One attempt has utilized curved ribs in the side housing circuit in the hope of increasing heat extraction since the ribs would function as a heat sink; such ribs were not contoured with the intent of varying the flow character, or varying the flow speed or even the flow volume distribution. Yet still other prior art attempts have introduced the oil cooling system adjacent the water cooling system with the hope that each would be capable of extracting heat at different rates at different portions of the epitrochoid surface. Although each of these prior art attempts have contributed some degree of improvement to the very severe problem of temperature variation, they have not been totally satisfactory from the standpoint of optimum fuel consumption and engine efficiency when viewed thermodynamically or mechanically. Specifically the prior art has been deficient in inappropriate variation of the flow velocity, improper character of the flow at various locations around the epitrochoid surface, little or no variation of the total volume of fluid between the separate housings. It is with this total combination of requirements in mind that the present invention has been developed.